The present invention relates to a flash memory device and a method of operating the same, more particularly, to a flash memory device and a method of operating the same being capable of compensating operating characteristics which change as the number of the operation increases.
As one kind of the memory devices, a non-volatile memory device has a characteristic that data stored in the memory cells are not erased even if a power supply is turned off. A flash memory device is a typical non-volatile memory device. The flash memory device may be classified into a NOR flash memory device and a NAND flash memory device based on a structure of a memory cell array. A gate of the flash memory cell has a structure comprising a tunnel insulating layer, a floating gate, a dielectric layer and a control gate.
In such a flash memory device (in particular, the NAND flash memory device), a programming operation and an erasing operation are performed through an F-N (Fowler-Nordheim) tunneling phenomenon. Electrons are accumulated on the floating gate by the programming operation, and the electrons accumulated on the floating gate are discharged to a substrate by the erasing operation. And, when a reading operation is performed, a threshold voltage of the memory cell, which is changed according to the amount of electrons accumulated on the floating gate, is detected, and the data is read by a level of the detected threshold voltage.
As the number of operation cycle (i.e., the number of times the above operation is carried out) increases, the threshold voltage of the memory cell in the programming state (or the erasing state) changes.
FIG. 1 is a characteristic graph showing a variation of threshold voltage according to the number of operation cycle of a flash memory device.
Referring to FIG. 1, as the number of operation cycle increases, the threshold voltage of the memory cell gradually increases even if the programming operation is performed under the same condition. The threshold voltage can also be increased at the time of performing the erasing operation. As the number of operation cycle increases, the electrons are trapped in the tunnel insulating layer, which can influence the programming operation.
If the threshold voltage is changed under the same conditions of the programming operation, the data stored in the memory cell is not read normally when the reading operation is performed, causing a malfunction of the memory cell.